The present invention relates to a mechanical pulley arrangement designed specifically to provide a mechanical linkage between the mechanical output of an automotive cruise control system and the vehicle's throttle.
In an electronic cruise control system which makes use of an electric actuator, such as is disclosed in U.S. Pat. No. 4,656,407, the mechanical output of an electric servo motor is mechanically coupled to the vehicle's throttle in order to modify the speed of the engine in accordance with the error signal produced by the control system. As described in Applicant's co-pending application referenced above, the rotation of the servo motor may be coupled via a clutch mechanism to a pulley having a cable attached to the throttle. Rotation of the pulley causes pivoting of the throttle element which, in turn, causes the butterfly valve of the carburetor to open and increase the speed of the engine. A return spring causes the throttle element to close the butterfly valve when tension is removed from the cable, thus reducing the speed of the engine.
One of the problems, however, with electronic cruise control systems is the non-linearity of the throttle control provided in most vehicles. Typically, as aforesaid, the throttle is connected to a butterfly valve located on the carburetor. This butterfly valve serves to control the volume of air/fuel mixture supplied to the engine and thus controls its speed. The valve is capable of movement through 90 degrees, with 0 degrees representing shutoff and 90 degrees representing full power. The non-linearity of this system can be appreciated when it is understood that the movement of the throttle butterfly valve from 0 to approximately 35 degrees represents 80% of the speed attainable by the engine. Thus, the faster a vehicle is operated the more movement of the butterfly valve is required to control the vehicle speed. The converse is also true. The slower a vehicle is driven, the less throttle butterfly movement is required. This non-linearity of the throttle is not a problem when the vehicle is operated manually since a human operator may easily compensate for such non-linearity by adjusting the pressure applied to the accelerator pedal in order to reach and maintain the desired speed.
An electronic cruise control system is, however, a linear one. This means that for a given error in speed the control system will respond with a given throttle correction, regardless of the speed the vehicle is traveling. The amount of correction supplied by the control system to the throttle for a given speed error is defined as the system sensitivity. It is desirable to have a system sensitivity that is low enough so that when the vehicle is operated at low speeds the correction supplied to the throttle by the control system is not so great as to cause surging. Surging occurs when the throttle correction supplied by the control system is great enough to cause the speed of the vehicle to alternately overshoot and undershoot the set point speed due to the throttle being operated in a very sensitive region of its response curve. Thus, to achieve a smooth and stable response, the system sensitivity must be low enough so as to never induce a surging condition in the vehicle. On the other hand, if the system sensitivity is too low, the accuracy and responsiveness of the control system will suffer. If the mechanical characteristics of the system are linear, the solution is to adjust the gain of the control system until the system sensitivity is neither too high nor too low. With a non-linear throttle control, however, the system sensitivity changes with the speed of the vehicle. This means that a gain producing an adequate system sensitivity at high speeds will cause surging of the vehicle at low speeds. Conversely, a gain producing an adequate system sensitivity at low speeds will result in slow and inaccurate control of vehicular speed at higher speeds. One solution to this problem is to design the electronic control system with a variable gain which varies inversely with the sensitivity of the throttle control as the speed of the vehicle changes. However, such non-linear electronic devices increase the cost and complexity of the electronic control system.
It is an object of the present invention to provide a simple and effective mechanical solution to this problem that will compensate for the non-linearity of the throttle by converting it to a near linear system thus matching it to the linear characteristics of the electronics.